push/pushf/pop/popf

The push instructions push the value of the given register onto the stack. pushf pushes the flags.
The pop instructions pop an 8 bit value from the stack and write it to the given register. popf pops the flags.

The flags are encoded in an 8 bit value as follows: 0000CVZS (the sign flag is the least significant bit).

Prefixing these instructions with prefix_a16 results in undefined behavior. (The values would be written to or read from IO devices.)

Instruction	Encoding	Semantics	Cycles
push reg81	00 rrr 100	$\mathit{sp} \gets \mathit{sp} - 1; \mathit{mem}[\mathit{sp}] \gets \mathit{reg8}$	4
pushf	00 110 100	$\mathit{sp} \gets \mathit{sp} - 1; \mathit{mem}[\mathit{sp}] \gets \mathit{flags}$	4
pop reg81	00 rrr 101	$\mathit{reg8} \gets \mathit{mem}[\mathit{sp}]; \mathit{sp} \gets \mathit{sp} + 1$	4
popf	00 110 101	$\mathit{flags} \gets \mathit{mem}[\mathit{sp}]; \mathit{sp} \gets \mathit{sp} + 1$	4

1. reg8 may not be [pi] (rrr = 110).